1. Field of the Invention
The present invention relates to an infrared solid-state image sensing apparatus and a manufacturing method therefor.
2. Related Background Art
A solid-state image sensing device, which has a plurality of picture elements comprised of photoelectric converting sections and charge transfer sections, is used not only for an industrial video camera but also extensively used for a consumer video camera. The photoelectric converting section has a different structure, depending on the type of light to be sensed. The charge transfer section is generally constituted by a charge coupled device (CCD) although some charge transfer sections are configured by wiring as in MOS solid-state image sensing devices.
An early CCD was a surface channel type wherein signal charges are transferred along the surface of a semiconductor substrate. This type of CCD was advantageous in that it was capable of transferring a large number of signal charges, but it was disadvantageous in that it had low transfer efficiency (the rate at which signal charges can be transferred without losing them). This disadvantage made this type undesirable for the solid-state image sensing device which has more than 350,000 picture elements and which is extensively used today. To solve the problem, a buried channel type CCD was developed. The buried channel type CCD uses, as the charge transferring passage, a region where an impurity of the opposite conducting type (the second conducting type) from that of the semiconductor substrate (the first conducting type) has been diffused (buried channel). With reverse bias applied between the aforesaid region and the semiconductor substrate, the voltage applied to a transfer electrode, which is disposed on the buried channel with an insulating film between, is changed gradually to store signal charges in the buried channel located apart from the surface of the semiconductor substrate, then the stored signal charges are transferred. The buried channel type CCD is ideally used for a solid-state image sensing device having many picture elements because it provides high transfer efficiency, although it can transfer fewer signal charges.
The following describes a conventional solid-state image sensing device which is used at room temperature to sense visible light (hereinafter referred to as "room temperature visible solid-state image sensing device"). FIG. 16 shows a conceptual top plan view of the room temperature visible solid-state image sensing device which is interline transfer type. Photoelectric converting sections 18 are disposed vertically and horizontally in a matrix pattern on a substrate. Adjoining the photoelectric converting sections, a plurality of vertical CCDs 19 are disposed to transfer signal charges, which have been photoelectrically converted through the photoelectric converting sections 18, in a vertical direction. One end of each vertical CCD 19 is connected to a horizontal CCD 20 which transfers the signal charges from the vertical CCD 19 in the horizontal direction. The positional relationship between the vertical and horizontal directions is relative; therefore, the directions mentioned here may be reversed by, for example, reversing the orientation of the device. Generally, however, a CCD adjoining the photoelectric converting section is called a vertical CCD and a CCD that transfers the signal charges from the vertical CCD is called a horizontal CCD. This commonly used nomenclature of these CCDs will be used throughout the present description.
Each CCD is provided with a buried channel, which is the opposite conducting type (second conducting type) from that of the substrate, and a transfer electrode (not shown) disposed on the buried channel with an insulating film between. The signal charges generated in the photoelectric converting section 18 are transferred in sequence by changing the voltage applied to the electrodes in sequence. The buried channel of the vertical CCD 19 (hereinafter referred to as "the first buried channel") and the buried channel of the horizontal CCD 20 (hereinafter referred to as "the second buried channel") are formed at the same time to simplify the process. The diffusion depth is 0.6 .mu.m or less for the solid-state image sensing device with more than 350,000 picture elements which is commonly used these days.
The driving method for the CCDs will now be described. The 4-phase driving method is usually employed for the vertical CCD 19 and the 2-phase driving method for the horizontal CCD 20 in order to improve the performance of the solid-state image sensing device and also to permit easier manufacture. In the 4-phase driving method, four transfer electrodes constitute one group, and four different pulses, which are different in time series, are applied to the respective transfer electrodes, then potential wells formed under the transfer electrodes are moved to transfer signal charges. In the 2-phase driving method, two different types of pulses are used to transfer signal charges. In general, these pulse signals are applied to two transfer electrodes to define the transfer direction of signal charges. Further, different heights are used for the potential wells formed under the two transfer electrodes. Thus, four transfer electrodes apparently constitute one group as in the case of the 4-phase driving method.
In recent years, the solid-state image sensing devices are not only used for image sensing in the visible region but also for image sensing in the infrared region as infrared solid-state image sensing devices which are expected to find a wide range of application fields including various industrial measurement, monitoring, medical treatment, space, and defense. In the infrared solid-state image sensing device, the photoelectric converting section 18 is formed by platinum silicide (PtSi) Schottky junction; the photoelectric converting section converts infrared rays into signal charges. The obtained signal charges are transferred to the vertical CCD 19 and the horizontal CCD 20 in sequence and read out. Specifically, the infrared solid-state image sensing devices directly employ the CCDs of the room temperature visible solid-state image sensing device without adding any change and they replace the photoelectric converting section 18 with a structure which enables them to sense infrared rays.
The following describes a conventional infrared solid-state image sensing apparatus in conjunction with the accompanying drawings. The infrared solid-state image sensing apparatus is defined as a Schottky type infrared solid-state image sensing apparatus. FIG. 17 gives a cross-sectional view of a Schottky-junction photoreceptor (photoelectric converting section) 76 and a charge reading section (charge transfer section) 77 in the conventional infrared solid-state image sensing apparatus. The photoreceptor 76 is comprised of a Schottky junction 64 of a silicon substrate 61 and a metallic silicide layer 63a. Specific materials for the metallic silicide layer include platinum silicide, palladium silicide, and iridium silicide. The metallic silicide layer 63a is formed by attaching metal to the silicon substrate 61 by evaporation and heat treatment, thereby producing chemical reaction between the silicon and the metal. At the same time when the metallic silicide layer 63a is formed, the Schottky junction 64 is formed on the interface between the silicon substrate 61 and the metallic silicide layer 63a. The Schottky junction 64 is usually surrounded by a guard ring 65 which controls the electric field intensity around the Schottky junction 64 to prevent leakage currents.
The charge reading section 77 adjoining the photoreceptor 76 is disposed on the silicon substrate 61. The charge reading section 77 is composed of a gate diffusion section 66 and a buried channel 68 which are regions where impurities have been diffused, and a transfer gate electrode 67 and a transfer electrode 69 which are disposed thereon with a silicon oxide film 70 between. The gate diffusion section 66 adjoins the guard ring 65, and the buried channel 68 is disposed adjacently to the gate diffusion section 66. Disposed on the gate diffusion section 66 and the buried channel 68 are the transfer gate electrode 67 and the transfer electrode 69, and the silicon oxide film 70 intervenes therebetween.
A reflection film 73 is generally disposed on the photoreceptor with an inter-layer insulating film 72 between which reflection film reflects infrared rays, which have been transmitted without being photoelectrically converted through the Schottky junction 64, and directs them into the Schottky junction 64 again, thus improving the sensitivity. A channel stop 75 is provided to prevent signal charges from leaking. The surface of the infrared imaging apparatus is covered with a protective film 74 to improve durability.
Infrared rays 71 entering through the rear surface of the silicon substrate 61 are photoelectrically converted by the Schottky junction 64 into signal charges which are stored in the photoreceptor 76. When voltage is applied to the transfer gate electrode 67, the stored signal charges are transferred to the buried channel 68 through the gate diffusion section 66. Applying voltage to a plurality of transfer electrodes 69 arranged vertically to the paper surface causes the signal charges to be transferred in sequence.
For the substrate, a silicon substrate and other semiconductor substrate made of germanium, etc. is used. The metal used for producing the Schottky junction differs according to the material of the substrate used. In the case of a germanium substrate, such metals as platinum, palladium, and iridium are used as in the case of the silicon substrate.
The infrared solid-state image sensing device is usually cooled to approximately the temperature of liquid nitrogen (77 kelvin) when it is used. The infrared solid state image device which is cooled for use is called "cooled infrared solid-state image sensing apparatus", which, for brevity, is referred to as an IR solid-state image sensing apparatus in this description.
A conventional IR solid-state image sensing apparatus, which is cooled to about the temperature of liquid nitrogen when it is used, posed a problem of low transfer efficiency of CCDs. There was also a problem known as "blooming" which refers to a phenomenon wherein, when a hot object is imaged and displayed as an image on a CRT or the like, the hot object is displayed larger than the actual size thereof (especially the lower side of the image). As a result, the hot object is distorted when it is displayed, making the image difficult to be seen.
The conventional IR solid-state image sensing apparatus suffered from another shortcoming in that the sensitivity of photoreceptors varied greatly, requiring the correction of signals to control the variation in the sensitivity of the photoreceptors.